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1. Field of the Invention
The present invention relates to methods and apparatus for alignment of high-resolution pixel images, and more particularly to methods and apparatus for aligning stroboscopic voltage contrast images of operating states of integrated circuit devices. The aligned images are useful in analyzing dynamic failures of integrated circuits, for example.
2. Description of Related Art
Decreases in the size of internal features of Very Large Scale Integrated (VLSI) circuit devices demand ever faster and more reliable design and testing. Conventional Integrated Circuit (IC) testers can only retrieve information from the external pins of the Device Under Test (DUT), thus limiting subsequent diagnoses. A failure detected with a conventional tester may be caused by a discrepancy at any point inside the component. If the DUT has hundreds of thousands of gates, fault identification becomes a complex and tedious chore.
In a typical test operation, a conventional IC tester (one which applies stimuli to input pins of the IC and measures the results at the IC output pins) detects a fault at some vector (for example, vector v) in a test vector sequence when testing an IC device. The test sequence may contain a large number of test vectors, for example, 100 or more test vectors, each vector representing a set of stimuli, such as input voltages, applied to the pins of the DUT. The origin of the detected fault occurs somewhere in the chip, at some vector (for example, vector a), between the first vector of the sequence (vector 1) and the vector at which the fault was detected (vector v). This fault then propagates forward and appears at an external pin or bond pad of the DUT at vectro v. It is then desired to identify the nature, location and time of generation of the fault occurring at vector a.
Internal probing of the chip is therefore necessary. For many years, the preferred solution has been to probe the chip using a low energy (around 1 keV) scanning electron microscope (SEM). See, for example, E. Menzel & E. Kubalek, Fundamentals of Electron Beam Testing of Integrated Circuits, 5 SCANNING 103-122 (1983), and E. Plies & J. Otto, Voltage Measurement Inside Integrated Circuit Using Mechanical and Electron Probes, IV SCANNING ELECTRON MICROSCOPY 1491-1500 (1985).
Until recently, the SEM was a sophisticated lab instrument, used only by experienced researchers. In 1987, the "IDS 5000.TM." workstation-based, electron-beam test system was commercially introduced by Schlumberger. S. Concina, G. Liu, L. Lattanzi, S. Reyfman & N. Richardson, Software Integration in a Workstation Based E-Beam Tester, INTERNATIONAL TEST CONFERENCE PROCEEDINGS (1986); N. Richardson, E-Beam Probing for VLSI Circuit Debug, VLSI SYSTEMS DESIGN (1987); S. Concina & N. Richardson IDS 5000: an Integrated Diagnosis System for VLSI, 7 MICROELECTRONIC ENGINEERING (1987). See also U.S. Pat. Nos. 4,706,019 and 4,721,909 to N. Richardson.
Measurement and information gathering is thus no longer a major problem in analyzing IC failures. Organizing the extensive and detailed information obtained about an IC with such a tester is, however, critical to rapid and effective fault diagnosis.
Rather than seeking the answer to an absolute problem ("Why does this device fail?"), it may be preferable to address a relative problem ("Where, when and why does this device behave differently from a known good device?"). With the latter approach to diagnosis, operating faults in the IC can be traced.
To perform such a diagnosis, a time period of interest between 0 (the beginning of the test sequence of interest) and the first fault detection is selected. This period is divided into intervals such that, in each interval, the IC has a fixed behavior. Each interval thus corresponds to an operating state of the IC. After the states of interest have been identified, a comparison can be made between a known "good" IC device and the DUT, at each state.
Such comparisons can be made with stroboscopic voltage contrast images. The device under test (DUT) is stimulated with a sequence of test "vectors" in a conventional E-Beam test system such as the Schlumberger "IDS 5000." Each test "vector" represents a specified set of stimuli, such as input voltages applied to the pins of the IC. During the application of each vector of the sequence, the DUT is in a state for a period of time called a "strobe window." By pulsing the electron beam repeatedly in a certain phase to the beginning of the sequence, a stroboscopic image representing the state of the DUT in any desired strobe window can be obtained. This strobe process is analogous to that of using a stroboscopic light to "freeze" the operation of an automobile engine to adjust ignition timing. A series of state images makes up a stack.
The process is repeated using a known good IC device (also called a golden die), so as to acquire a stack of stroboscopic images representing its states in response to the same series of test vectors. Each of the images may, for example, be a graphical representation in digital format in the form of a 512.times.512 matrix of pixels of varying intensity, the intensities being represented by a value between 0 and 255 (an 8-bit value). The images thus acquired, representing operating states of the DUT, can be stored and used to diagnose operating faults in the DUT.
After the images have been acquired, the stacks may be compared. The comparison may take the form of substracting an image of the golden die from an image of the DUT (or vice versa), pixel by pixel, where the compared images are those produced in response to the same set of stimuli. The comparison is repeated, image by image, so as to produce a stack of "difference" images.
Alternatively, the two stacks may represent states of a single DUT in response to two different sets of stimuli. For example, a DUT may operate as designed at a given temperature or with a given input voltage, but fail at higher temperature or input voltage. One stack of images may be acquired representing correct operation of the DUT under one set of circumstances and a second stack acquired representing failed operation under a second set of circumstances. These stacks may also be compared, image by image, to produce a stack of "difference" images.
If the good and faulty devices behave the same way, the images are the same. Likewise, if a device subjected to different sets of stimuli is operating the same way in response to the different sets of stimuli, the images are the same. Any divergence between the two sets of images may be considered as a discrepancy in the test (or failing) device. T. May, G. Scott, E. Meieran, P. Wiener & V. Rao, Dynamic Fault Imaging of VLSI Random Logic Devices, INTERNATIONAL PHYSICS SYMPOSIUM PROCEEDINGS (1984). On a one-image-per-state basis, the comparison process reveals the propagation of the fault from its origin to the place where is first detected. Such a comparison process is known as Dynamic Fault Imaging (DFI).
Of course, for the comparison to work properly, the two stacks of images must represent identical conditions: the same area of the chip, imaged at the same magnification under the same SEM operating conditions. However, it is possible that the images of one stack are translated or rotated with respect to those of another stack due to slightly different orientation of the chips with respect to the SEM when the images are acquired. Perfect alignment is not always possible.
Therefore a spatial "warping" operation may need to be performed on each image of one stack to align it with a corresponding image of the other stack. The warping operation must result in precise alignment because the comparison (difference) of two images is made pixel by pixel. Any misalignment would appear in the resulting (differenced) image. A warping procedure offering such precision could take an unacceptably long time (tens of seconds) for use in rapid, interactive DFI diagnosis if there is no dedicated hardware to perform the warping operation.
Once the images have been acquired, filtered, aligned and compared pixel-by-pixel to create a stack of resulting "difference" images, the difference images may be displayed to permit tracking of fault propagation through the series of images. The "difference" images may also be processed to enhance only the important information (the fault propagation) for analysis.
To perform the DFI process efficiently demands rapid and precise alignment of images to be compared, so that when the images (or their differences) are displayed, propagation of a fault in the DUT can be readily traced, even by a person having little knowledge of the functionality of the device.
Some operations in a DFI session are repetitive (e.g., image acquisition and processing). For instance, once the right SEM operating conditions are found for an image acquired in a controlled manner, the same operation can be repeated in a batch process for the rest of the images in the stack. It is desirable to automate such repetitive tasks to allow the engineer to concentrate on the diagnostic work. At the same time, interactivity is desired so that the engineer may set parameters for the task and view the result without significant delay.
Prior art methods are known for aligning images. For example, a method which has heretofore been carried out in the Schlumberger "IDS 5000" system involves selection of three locations from a first image to be aligned respectively with three selected locations of a second image, calculating a geometric planar transformation in the form of a matrix of floating point values, and using the matrix values to transform positions of the first image into positions of a resultant image having pixels aligned with pixels of the second image, using floating point arithmetic operations. While the quality of the alignment is acceptable, the processing is not as rapid as desired. To achieve "interactive" operation, it is desirable to carry out any selected process, such as image alignment, and display the result within, for example, a period of not more than two seconds.
Other imaging systems are known in the art which employ convolutions to perform geometric operations, filtering and pattern recognition. Such systems sometimes use fast Fourier transform to speed up the computation of large convolutions. Floating-point arithmetic is often used since it is not known in advance how many operations will be performed on the data.
Another way to speed up image processing is to employ dedicated hardware processors designed to perform specific tasks at high speed. The cost of such processors can, however, add significantly to the cost of a test system. It is preferred instead to use the general-purpose processor of a standard engineering workstation to perform image processing, if this can be done without much loss of processing time.
A principal aim of the present invention is to provide a method of carrying out the alignment of stroboscopic voltage contrast images in a workstation of the type provided in a system such as the Schlumberger "IDS 5000" with sufficient speed to permit Dynamic Fault Imaging with an interactive "feel," but without the need for dedicated hardware to perform the processing.
More broadly, it is an object of the present invention to provide methods and apparatus for aligning features of one image pixel-for-pixel with those of another image.
Yet another object of the present invention is to provide methods and apparatus for aligning stroboscopic voltage contrast images representing integrated circuit operating states, as an aid in DFI analysis of integrated circuits.
Still another object of the present invention to provide for rapid and interactive alignment of voltage contrast images in a general-purpose workstation associated with an electron-beam test system, without the need for special-purpose computing hardware to perform the alignment.
These and other objects of the present invention will become apparent from the following detailed description of the preferred embodiments and the accompanying drawings.